diff --git a/configure.ac b/configure.ac
index 695a86b..17bd5f9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -158,9 +158,14 @@ AC_CHECK_HEADERS([sys/param.h])
 # Checks for global variable declarations.
 AC_CHECK_DECLS([optreset])
 
+# Checks for structures.
+AC_CHECK_MEMBERS([struct tm.tm_gmtoff],,,[
+#include <time.h>
+])
+
 # Checks for library functions.
 AC_FUNC_STRERROR_R
-AC_CHECK_FUNCS([localtime_r gmtime_r])
+AC_CHECK_FUNCS([localtime_r gmtime_r timegm _mkgmtime])
 AC_CHECK_FUNCS([getopt_long])
 
 # Versioning.
diff --git a/examples/output_xml.c b/examples/output_xml.c
index 2baa329..86a3957 100644
--- a/examples/output_xml.c
+++ b/examples/output_xml.c
@@ -230,9 +230,16 @@ dctool_xml_output_write (dctool_output_t *abstract, dc_parser_t *parser, const u
 		goto cleanup;
 	}
 
-	fprintf (output->ostream, "<datetime>%04i-%02i-%02i %02i:%02i:%02i</datetime>\n",
-		dt.year, dt.month, dt.day,
-		dt.hour, dt.minute, dt.second);
+	if (dt.timezone == DC_TIMEZONE_NONE) {
+		fprintf (output->ostream, "<datetime>%04i-%02i-%02i %02i:%02i:%02i</datetime>\n",
+			dt.year, dt.month, dt.day,
+			dt.hour, dt.minute, dt.second);
+	} else {
+		fprintf (output->ostream, "<datetime>%04i-%02i-%02i %02i:%02i:%02i %+03i:%02i</datetime>\n",
+			dt.year, dt.month, dt.day,
+			dt.hour, dt.minute, dt.second,
+			dt.timezone / 3600, (dt.timezone % 3600) / 60);
+	}
 
 	// Parse the divetime.
 	message ("Parsing the divetime.\n");
diff --git a/include/libdivecomputer/datetime.h b/include/libdivecomputer/datetime.h
index d8f596b..768883d 100644
--- a/include/libdivecomputer/datetime.h
+++ b/include/libdivecomputer/datetime.h
@@ -26,6 +26,8 @@
 extern "C" {
 #endif /* __cplusplus */
 
+#define DC_TIMEZONE_NONE 0x80000000
+
 #if defined (_WIN32) && !defined (__GNUC__)
 typedef __int64 dc_ticks_t;
 #else
@@ -39,6 +41,7 @@ typedef struct dc_datetime_t {
 	int hour;
 	int minute;
 	int second;
+	int timezone;
 } dc_datetime_t;
 
 dc_ticks_t
diff --git a/src/atomics_cobalt_parser.c b/src/atomics_cobalt_parser.c
index e43243c..e16077b 100644
--- a/src/atomics_cobalt_parser.c
+++ b/src/atomics_cobalt_parser.c
@@ -122,6 +122,7 @@ atomics_cobalt_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *dateti
 		datetime->hour   = p[0x18];
 		datetime->minute = p[0x19];
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/citizen_aqualand_parser.c b/src/citizen_aqualand_parser.c
index f89eacf..319dcf7 100644
--- a/src/citizen_aqualand_parser.c
+++ b/src/citizen_aqualand_parser.c
@@ -95,6 +95,7 @@ citizen_aqualand_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *date
 		datetime->hour   = bcd2dec(p[0x0A]);
 		datetime->minute = bcd2dec(p[0x0B]);
 		datetime->second = bcd2dec(p[0x0C]);
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/cochran_commander_parser.c b/src/cochran_commander_parser.c
index 33f3b28..12e8b42 100644
--- a/src/cochran_commander_parser.c
+++ b/src/cochran_commander_parser.c
@@ -435,6 +435,7 @@ cochran_commander_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *dat
 			datetime->day = data[layout->datetime + 2];
 			datetime->month = data[layout->datetime + 5];
 			datetime->year = data[layout->datetime + 4] + (data[layout->datetime + 4] > 91 ? 1900 : 2000);
+			datetime->timezone = DC_TIMEZONE_NONE;
 			break;
 		case DATE_ENCODING_SMHDMY:
 			datetime->second = data[layout->datetime + 0];
@@ -443,6 +444,7 @@ cochran_commander_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *dat
 			datetime->day = data[layout->datetime + 3];
 			datetime->month = data[layout->datetime + 4];
 			datetime->year = data[layout->datetime + 5] + (data[layout->datetime + 5] > 91 ? 1900 : 2000);
+			datetime->timezone = DC_TIMEZONE_NONE;
 			break;
 		case DATE_ENCODING_TICKS:
 			ts = array_uint32_le(data + layout->datetime) + COCHRAN_EPOCH;
diff --git a/src/cressi_edy_parser.c b/src/cressi_edy_parser.c
index c9e1d91..eaf55c3 100644
--- a/src/cressi_edy_parser.c
+++ b/src/cressi_edy_parser.c
@@ -115,6 +115,7 @@ cressi_edy_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime)
 		datetime->hour = bcd2dec (p[14]);
 		datetime->minute = bcd2dec (p[15]);
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/cressi_leonardo_parser.c b/src/cressi_leonardo_parser.c
index b8c47e0..d7cf0de 100644
--- a/src/cressi_leonardo_parser.c
+++ b/src/cressi_leonardo_parser.c
@@ -100,6 +100,7 @@ cressi_leonardo_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datet
 		datetime->hour = p[11];
 		datetime->minute = p[12];
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/datetime.c b/src/datetime.c
index 9d07f8c..3ceed42 100644
--- a/src/datetime.c
+++ b/src/datetime.c
@@ -61,6 +61,47 @@ dc_gmtime_r (const time_t *t, struct tm *tm)
 #endif
 }
 
+static time_t
+dc_timegm (struct tm *tm)
+{
+#if defined(HAVE_TIMEGM)
+	return timegm (tm);
+#elif defined (HAVE__MKGMTIME)
+	return _mkgmtime (tm);
+#else
+	static const unsigned int mdays[] = {
+		0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
+	};
+
+	if (tm == NULL ||
+		tm->tm_mon  < 0 || tm->tm_mon  > 11 ||
+		tm->tm_mday < 1 || tm->tm_mday > 31 ||
+		tm->tm_hour < 0 || tm->tm_hour > 23 ||
+		tm->tm_min  < 0 || tm->tm_min  > 59 ||
+		tm->tm_sec  < 0 || tm->tm_sec  > 60)
+		return (time_t) -1;
+
+	/* Number of leap days since 1970-01-01. */
+	int year = tm->tm_year + 1900 - (tm->tm_mon < 2);
+	int leapdays =
+		((year / 4) - (year / 100) + (year / 400)) -
+		((1969 / 4) - (1969 / 100) + (1969 / 400));
+
+	time_t result = 0;
+	result += (tm->tm_year - 70) * 365;
+	result += leapdays;
+	result += mdays[tm->tm_mon];
+	result += tm->tm_mday - 1;
+	result *= 24;
+	result += tm->tm_hour;
+	result *= 60;
+	result += tm->tm_min;
+	result *= 60;
+	result += tm->tm_sec;
+	return result;
+#endif
+}
+
 dc_ticks_t
 dc_datetime_now (void)
 {
@@ -72,11 +113,23 @@ dc_datetime_localtime (dc_datetime_t *result,
                        dc_ticks_t ticks)
 {
 	time_t t = ticks;
+	int offset = 0;
 
 	struct tm tm;
 	if (dc_localtime_r (&t, &tm) == NULL)
 		return NULL;
 
+#ifdef HAVE_STRUCT_TM_TM_GMTOFF
+	offset = tm.tm_gmtoff;
+#else
+	struct tm tmp = tm;
+	time_t t_local = dc_timegm (&tmp);
+	if (t_local == (time_t) -1)
+		return NULL;
+
+	offset = t_local - t;
+#endif
+
 	if (result) {
 		result->year = tm.tm_year + 1900;
 		result->month = tm.tm_mon + 1;
@@ -84,6 +137,7 @@ dc_datetime_localtime (dc_datetime_t *result,
 		result->hour = tm.tm_hour;
 		result->minute = tm.tm_min;
 		result->second = tm.tm_sec;
+		result->timezone = offset;
 	}
 
 	return result;
@@ -106,6 +160,7 @@ dc_datetime_gmtime (dc_datetime_t *result,
 		result->hour = tm.tm_hour;
 		result->minute = tm.tm_min;
 		result->second = tm.tm_sec;
+		result->timezone = 0;
 	}
 
 	return result;
@@ -124,7 +179,15 @@ dc_datetime_mktime (const dc_datetime_t *dt)
 	tm.tm_hour = dt->hour;
 	tm.tm_min = dt->minute;
 	tm.tm_sec = dt->second;
-	tm.tm_isdst = -1;
+	tm.tm_isdst = 0;
 
-	return mktime (&tm);
+	time_t t = dc_timegm (&tm);
+	if (t == (time_t) -1)
+		return t;
+
+	if (dt->timezone != DC_TIMEZONE_NONE) {
+		t -= dt->timezone;
+	}
+
+	return t;
 }
diff --git a/src/diverite_nitekq_parser.c b/src/diverite_nitekq_parser.c
index 6c7a05f..dce40d8 100644
--- a/src/diverite_nitekq_parser.c
+++ b/src/diverite_nitekq_parser.c
@@ -118,6 +118,7 @@ diverite_nitekq_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datet
 		datetime->hour = p[3];
 		datetime->minute = p[4];
 		datetime->second = p[5];
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/hw_ostc_parser.c b/src/hw_ostc_parser.c
index b23e6d3..f1c656f 100644
--- a/src/hw_ostc_parser.c
+++ b/src/hw_ostc_parser.c
@@ -394,6 +394,7 @@ hw_ostc_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime)
 	dt.hour   = p[3];
 	dt.minute = p[4];
 	dt.second = 0;
+	dt.timezone = DC_TIMEZONE_NONE;
 
 	if (version == 0x24) {
 		if (datetime)
@@ -405,8 +406,10 @@ hw_ostc_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime)
 
 		ticks -= divetime;
 
-		if (!dc_datetime_localtime (datetime, ticks))
+		if (!dc_datetime_gmtime (datetime, ticks))
 			return DC_STATUS_DATAFORMAT;
+
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/mares_darwin_parser.c b/src/mares_darwin_parser.c
index 1201ec6..ba0382f 100644
--- a/src/mares_darwin_parser.c
+++ b/src/mares_darwin_parser.c
@@ -118,6 +118,7 @@ mares_darwin_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime
 		datetime->hour   = p[4];
 		datetime->minute = p[5];
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/mares_iconhd_parser.c b/src/mares_iconhd_parser.c
index 0c45a15..76324f3 100644
--- a/src/mares_iconhd_parser.c
+++ b/src/mares_iconhd_parser.c
@@ -329,6 +329,7 @@ mares_iconhd_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime
 		datetime->day    = array_uint16_le (p + 4);
 		datetime->month  = array_uint16_le (p + 6) + 1;
 		datetime->year   = array_uint16_le (p + 8) + 1900;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/mares_nemo_parser.c b/src/mares_nemo_parser.c
index 9a6cbfd..c0ab03b 100644
--- a/src/mares_nemo_parser.c
+++ b/src/mares_nemo_parser.c
@@ -200,6 +200,7 @@ mares_nemo_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime)
 		datetime->hour   = p[3];
 		datetime->minute = p[4];
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/oceanic_atom2_parser.c b/src/oceanic_atom2_parser.c
index 9133984..19abadb 100644
--- a/src/oceanic_atom2_parser.c
+++ b/src/oceanic_atom2_parser.c
@@ -324,6 +324,7 @@ oceanic_atom2_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetim
 			break;
 		}
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 
 		// Convert to a 24-hour clock.
 		datetime->hour %= 12;
diff --git a/src/oceanic_veo250_parser.c b/src/oceanic_veo250_parser.c
index ba69723..a3acf57 100644
--- a/src/oceanic_veo250_parser.c
+++ b/src/oceanic_veo250_parser.c
@@ -121,6 +121,7 @@ oceanic_veo250_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *dateti
 		datetime->hour   = p[3];
 		datetime->minute = p[2];
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 
 		if (parser->model == VEO200 || parser->model == VEO250)
 			datetime->year += 3;
diff --git a/src/oceanic_vtpro_parser.c b/src/oceanic_vtpro_parser.c
index 9c85854..894b626 100644
--- a/src/oceanic_vtpro_parser.c
+++ b/src/oceanic_vtpro_parser.c
@@ -135,6 +135,7 @@ oceanic_vtpro_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetim
 		}
 		datetime->minute = bcd2dec (p[0]);
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 
 		// Convert to a 24-hour clock.
 		datetime->hour %= 12;
diff --git a/src/shearwater_predator_parser.c b/src/shearwater_predator_parser.c
index c923153..4217f54 100644
--- a/src/shearwater_predator_parser.c
+++ b/src/shearwater_predator_parser.c
@@ -202,6 +202,8 @@ shearwater_predator_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *d
 	if (!dc_datetime_gmtime (datetime, ticks))
 		return DC_STATUS_DATAFORMAT;
 
+	datetime->timezone = DC_TIMEZONE_NONE;
+
 	return DC_STATUS_SUCCESS;
 }
 
diff --git a/src/suunto_d9_parser.c b/src/suunto_d9_parser.c
index 68e79f9..6c96fa0 100644
--- a/src/suunto_d9_parser.c
+++ b/src/suunto_d9_parser.c
@@ -317,6 +317,7 @@ suunto_d9_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime)
 			datetime->month  = p[5];
 			datetime->day    = p[6];
 		}
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/suunto_eon_parser.c b/src/suunto_eon_parser.c
index 066c997..2106c69 100644
--- a/src/suunto_eon_parser.c
+++ b/src/suunto_eon_parser.c
@@ -178,6 +178,7 @@ suunto_eon_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime)
 			datetime->minute = bcd2dec (p[4]);
 		}
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/suunto_eonsteel_parser.c b/src/suunto_eonsteel_parser.c
index baa0319..0ebee01 100644
--- a/src/suunto_eonsteel_parser.c
+++ b/src/suunto_eonsteel_parser.c
@@ -1114,7 +1114,11 @@ suunto_eonsteel_parser_get_datetime(dc_parser_t *parser, dc_datetime_t *datetime
 	if (parser->size < 4)
 		return DC_STATUS_UNSUPPORTED;
 
-	dc_datetime_gmtime(datetime, array_uint32_le(parser->data));
+	if (!dc_datetime_gmtime(datetime, array_uint32_le(parser->data)))
+		return DC_STATUS_DATAFORMAT;
+
+	datetime->timezone = DC_TIMEZONE_NONE;
+
 	return DC_STATUS_SUCCESS;
 }
 
diff --git a/src/suunto_vyper_parser.c b/src/suunto_vyper_parser.c
index 0ca8eaa..80f7d54 100644
--- a/src/suunto_vyper_parser.c
+++ b/src/suunto_vyper_parser.c
@@ -223,6 +223,7 @@ suunto_vyper_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime
 		datetime->hour   = p[3];
 		datetime->minute = p[4];
 		datetime->second = 0;
+		datetime->timezone = DC_TIMEZONE_NONE;
 	}
 
 	return DC_STATUS_SUCCESS;
diff --git a/src/uwatec_smart_parser.c b/src/uwatec_smart_parser.c
index 99bfa28..54bdee5 100644
--- a/src/uwatec_smart_parser.c
+++ b/src/uwatec_smart_parser.c
@@ -710,6 +710,8 @@ uwatec_smart_parser_get_datetime (dc_parser_t *abstract, dc_datetime_t *datetime
 
 		if (!dc_datetime_gmtime (datetime, ticks))
 			return DC_STATUS_DATAFORMAT;
+
+		datetime->timezone = utc_offset * 900;
 	} else {
 		// For devices without timezone support, the current timezone of
 		// the host system is used.